The present invention relates to an apparatus for automatic wire bonding, in particular wedge bonding using aluminium wire, which apparatus includes means for monitoring, during the bonding process, the quality of the bond between the wire and the surface to which it is to be bonded.
Wire bonding is the process of making electrical connections in semiconductor components by means of fine metal wire, typically wire with a diameter of from 12 microns to 500 microns. Examples of electrical connections which can be made using wire bonding techniques include connections between the contact surfaces of discrete or integrated chips and the contact leads of their packages, and, in the case of hybrid circuits, the connections between inserted monolithic elements and the film circuit which contains them.
A number of wire bonding techniques have been developed, and one which has been particularly successful is a microwelding technique using ultrasound. An automatic wire bonding apparatus on which such a technique can be operated is described in German Patent Application No. P 33 43 738. Aluminium wire, in contact with the contact surface to which it is to be bonded, is moved vigorously in the direction of the surface to which it is to be bonded so that its oxide layer breaks open. The wire is then subjected to pressure, and a permanent junction is created between the two materials. Motion of the wire is generated by an ultrasonic transducer excited by an ultrasonic generator to produce high-frequency mechanical vibrations.
In the particular wire bonding process known as wedge bonding, the ultrasonic energy is supplied in the range of 1 to 50 watts, depending on the wire size used. The ultrasonic energy is directed to the aluminium wire by a special tool known as a "wedge". The wire is fed through a hole at the bottom of the wedge. When the wedge with the aluminium wire touches the surface to which the wire is to be bonded, movement is stopped. The wire is pressed down with a small defined force, known as the bonding weight, and the wire is slightly deformed. This small deformation is known as the "pre-deformation". Ultrasonic energy is now switched on, and the welding process starts. During this time, the diameter of the aluminium wire is reduced by a few microns, the actual reduction depending on the size, physical properties and the precise chemical nature of the wire.
It is important in an automatic wire bonding apparatus to have as much control as possible over the process, and to be able to determine whether or not a bond has been successfully made. In particular, it is important to be able to ascertain when the wedge with the aluminium wire touches the surface to which the wire is to be bonded, so that movement of the wedge can be stopped. It would also be very useful for the operator of the wire bonding apparatus to ascertain whether a bond has been successfully made at the time of bonding rather than during a subsequent test routine. Because of the very rapid throughput of an automatic wire bonding apparatus, it would be advantageous if the bonding could be monitored immediately at the time of bonding, so that after the formation of an unsatisfactory bond the process can be stopped and the bonding conditions checked to prevent the production of a large number of unsatisfactory bonds, with the consequent wastage of time and expensive components and materials.
Most wire bonding machines currently in commercial use are only able to check whether a successful bond has been made after bonding is completed, using a test known as the loop-pull test. This test is typically used as a destructive test method in which samples are tested to destruction by pulling the loop between two bonds and noting the breaking force which is required, and the point at which the break occurs. In general, bonding is considered to be satisfactory if the wire breaks at the point approximately equidistant between the two bonds at which the force is applied; if the break occurs at the bond itself, with the wire lifting away from the surface to which it was supposed to be bonded, then this is due to the bonding being insufficient. If, alternatively, the wire breaks close to the bond, at the so-called heel, then this is generally the result of over-bonding, when too much pressure or too much ultrasonic energy has been applied to the wire, and the wire has been too highly deformed.
A number of methods have been proposed to check at the time of bonding whether or not a successful bond has been produced, but none of these proposed methods is totally successful, or suitable for the full range of bonds which the apparatus is required to make. In addition, these proposed methods have required expensive equipment.
One method which has been proposed is to determine whether there is an electrical contact between the aluminium wire and the surface to which it is to be bonded. This method can only be used when the surface is one which can be connected to a definite voltage. In practice, this means that the method can only be used where bonding is to a lead frame.
In an alternative proposal, data from a number of good bonds--frequency and current of the ultrasonic generator used for welding--is sampled electronically, and reference values are produced, against which similar data from all subsequent bonds is compared. This method works well with units with reproducible manufacturing parameters such as lead frames, but is not suitable in commercial hybrid applications where components and bonds have broad tolerances.
Alternative proposals include measuring the amount of aluminium wire used, and measuring the deflection of the transducer, which converts electrical signals to mechanical movement, during bonding. Neither of these proposals have proved satisfactory in practice.